1. Field of the Invention
This invention relates generally to generation and transmission of electrical energy by utilities. More particularly, the present invention relates to electrical energy storage apparatus that stores electrical energy during times of excess electrical production, and supplies the stored electrical energy during times of high demand.
2. Description of Related Art
Utility grids include transmission lines that transmit electrical energy from generating facilities to electrical power users. During the course of a day, the demands upon a utility grid vary greatly, dependent upon the energy users, the time of year, and the ready availability of energy. Generally, energy demands are greater in the morning when people are waking and heating or cooling their houses, and in the evening when they return home and begin cooking and other activities. In an industrial environment, demand may highest during the work day.
The amount of power available from the grid is usually determined by the generating capacity of the utility companies connected to the grid. The generating capacity should at least meet peak demand. Preferably, capacity should even exceed peak demand by a comfortable margin. In comparison to the large demand during the daily peak hours, during off-peak hours the energy demand may be very much smaller. Therefore, excess energy must be available during the daily off-peak hours, to supply the desired energy during peak times. Maintaining this excess capacity is expensive and wasteful in capital costs, manpower costs, and energy costs. Power plants may be held on standby during the off-peak hours. Unused energy may be dissipated without any useful purpose. Larger power plants must be built than those that would be required if only average power needs were addressed. And new power plant construction raises difficult environmental issues that add time, complexity, and increase costs.
For any utility grid there will be a specific limit to the generating capacity. In the unfortunate instance when users require more power than that available from the grid, the power company may choose to simply allow the power to degrade for all the users, thus causing a "brown-out". Alternately, the power company may decide to ration power among the users, cutting power to some while supplying substantially full power to others. This problem may be caused by unexpectedly high demand, for example during a hot spell when everyone is operating their air conditioners to keep cool. Also, this problem may be caused if, for example, a generating facility were to unexpectedly go off line. Generating facilities may go off line for any of a number of reasons, including scheduled maintenance as well as unexpected failures.
It would be an advantage if the energy that is available during off-peak times could be stored and then utilized later during the peak hours. If such a device were available, it would reduce the generating capacity needs of the utility grid, with substantial cost savings as a result. Furthermore, it would be an advantage of the reactive power could be controlled as well as the real power. Control of the reactive power provides a benefit in the form of correction of leading or lagging power factors that may be placed on the utility grid by users, particularly industrial users that have large machine shops which place large inductive loads on the line. The inductive loads cause a lagging current that degrades the quality of power, and stresses transmission lines, transformers, and other electrical apparatus.
It would be a further advantage if the energy storage were efficient and could be contained in a relatively small package. Power utilities would benefit from reduced generating costs. Furthermore, the environmental costs associated with generating electrical power would be avoided.